Many renewable energy sources produce direct current, for example, solar photovoltaic and chemical batteries. Direct current (DC) is converted into sinusoidal alternating current (AC) having a fixed frequency via an inverter, and the AC is transmitted to an electric grid or is used in a grid-disconnected way.
The inverter in a solar photovoltaic generation system recently trends to employ distributed micro-inverters. The micro-inverter may provide maximum power point control for each DC photovoltaic assembly, such that each DC photovoltaic assembly can produce a maximum energy, thereby improving the performance of the whole solar photovoltaic generation system. Furthermore, the micro-inverter may also generate a low AC voltage output, rather than a high DC voltage output of a centralized inverter system, so that the security and efficiency of the system can be improved.
FIG. 2 is a structural schematic diagram of a single-phase inverter of the prior art. As shown, the single-phase inverter 200 may comprise a DC-DC conversion circuit 201 and a DC-AC conversion circuit 202, with a storage capacitor 203 disposed at the DC input terminal so as to reduce ripple voltage.
FIG. 3 is a schematic diagram of a circuit structure of a single-phase inverter having flyback full bridge topology in the prior art. As shown, the single phase inverter 300 may comprise a DC-DC conversion circuit 301 and a DC-AC conversion circuit 302. The DC-DC conversion circuit 301 is used for controlling MPPT (Maximum Power Point Tracking) and generating sinusoidal wave to output a semi-sinusoidal wave. The DC-DC conversion circuit 301 may comprise a storage capacitor 303, a current detection element 304, a voltage detection element 305 and a flyback circuit 306. The flyback circuit 306 may further comprise a transformer T, a switch valve Q and a diode D. The main coil of the transformer T is connected with the switch valve Q in series and the secondary coil thereof and the diode D are connected to the output in series. Take a single flyback as an example here. Two or more interleaved flyback can also be used. The DC-AC conversion circuit 302 is H full bridge operating at a power frequency so as to invert semi-cycle sinusoidal wave and form a complete sinusoidal wave. And it can employ low-frequency, low-power consumption element, e.g. thyristor, etc.
One basic feature of a single phase inverter is as follows: an energy transmission between a power supply and a load comprises average energy and ripple of double frequency. The inverter wants to obtain DC having no ripple from a DC power supply and then transmits the averaged energy and ripple energy to the load, so that it is required that there is an energy storage unit in the inverter to handle the ripple energy. FIG. 1 is a waveform schematic of ripple power at DC terminals of a single-phase inverter in the prior art. As shown, the inverter generates an output power in-phase with AC grid energy, so that the output energy is oscillated between zero and a peak output power. When the output power of the inverter is zero, the current of photovoltaic assembly does not flow through the inverter, thus charging the storage capacitor; when the output power of the inverter is a peak value, the storage capacitor discharges to supplement power for the photovoltaic assembly, and thus the peak value goes to twice of the average. Thus, charging and discharging of the storage capacitor provide an additional AC component over the DC provided by the photovoltaic assembly, which is referred to as ripple power.
In order to manage ripple power having a double frequency, energy should be stored and released at a double frequency. To avoid a large voltage ripple caused by energy changing, a large capacitor is needed. Generally, an inverter employs in DC line a large capacity electrolytic capacitor as a passive filter. However, an electrolytic capacitor has various failure modes, and especially, a ripple current leads to self-heating inside the capacitor, thereby reducing the lifetime. An active filter circuit is widely studied to replace passive methods, which provides another ripple that counteracts double frequency ripple power by a separate energy conversion circuit. However, the method needs complex circuit and control method.
Thus, there is a need for simply eliminating ripple power at DC side in a photovoltaic grid-connected inverter.